This study aimed to determine whether a text message is as good as a postal letter as an invitation method for previous screenees in a breast cancer screening program, considering a non-inferiority margin of -2 percent points on participation rate. A non-inferiority randomized control trial was conducted. Women in the intervention group (n=5,362) were invited by text message, and women in the control group (n=5,482) were invited by letter, which is the standard invitation procedure of the program. In both groups, the invitation included a fixed appointment for mammography and a text message reminder 96 hours before the appointment. The primary outcome was screening participation rate (completing mammography within 12 weeks of invitation). The intention-to-treat analysis showed a participation rate of 87.3% and 86.6% in the control and intervention groups, respectively. The difference in participation rate was -0.7 percentage points (95% confidence interval [CI], -1.8 to ∞), indicating non-inferiority of text messages compared to letter invitations. The per-protocol analysis showed similar results. In conclusion, we found that a text message invitation for women who had previously participated in breast cancer screening was not inferior to the standard letter.